<html xmlns:wicket="http://wicket.apache.org">
<wicket:head>
    <style>
        div.my-tree div.wicket-tree-table-body {
            height: 20em;
            overflow: auto;
        }       
    </style>
</wicket:head>
<wicket:extend>
    <div wicket:id="treeTable" class = "my-tree"></div>
    <div wicket:id="addWindow"></div>
    <a href="#" wicket:id="addLink" class="button-link">Add new competences</a>
</wicket:extend>
</html>